A few questions here:

1. mime settings have some contradictions:

   [m.requestSerializer setValue:@"multipart/form-data" forHTTPHeaderField:@"Content-Type"];

   [formData appendPartWithFileData:imageData name:[NSString stringWithFormat:@"img.img%d", i+1] fileName:@"image.png" mimeType:@"image/png"];

   You have set two types of mime in these two places,multipart/form-data和image/png. Which one is correct? Need to communicate with the backend.

2. Contact the backend, have you received your request? What content is requested? What did he return? This error seems to be usually reported when parsing json. Maybe the background does not return valid json. In addition, your request is to upload multiple files at once. I don’t know if your backend interface supports it. You can try uploading only one picture.

3. There is no need to adjust the AFNetworking blockdispatch_async(dispatch_get_main_queue()...It has already helped you return to the main thread for execution.

Can I print out the data of imageData?

Have you solved it?
I also encountered the report The data couldn’t be read because it isn’t in the correct format.
But in fact the image was uploaded successfully
Client code: - (IBAction)upload:(id)sender {

AFHTTPSessionManager *manager = [AFHTTPSessionManager manager]; NSString *url3 = @"http://127.0.0.1:5000/upload";

[manager POST:url3 parameters:nil constructingBodyWithBlock:^(id _Nonnull formData) {

UIImage *image = [UIImage imageNamed:@"star"]; NSData *imageData = UIImagePNGRepresentation(image); NSString *fileName = @"star.png"; [formData appendPartWithFileData:imageData name:@"file" fileName:fileName mimeType:@"image/png"]; } progress:^(NSProgress *_Nonnull uploadProgress) { } success:^(NSURLSessionDataTask *_Nonnull task, id _Nullable responseObject) { NSLog(@"upload completed：%@",responseObject); } failure:^(NSURLSessionDataTask *_Nullable task, NSError * _Nonnull error) { NSLog(@"upload failed"); NSLog(@"%@",error.localizedDescription); }];

}

Backend code (python):
class receive(restful.Resource):

def post(self): file = request.files['file'] file.save(os.path.join(app.config['UPLOAD_FOLDER'], 'star2.png')) return 'upload completed'

api.add_resource(receive, '/upload', methods=['POST'])